1. Field of the Invention
The invention relates to quantitative determination of elements in composite materials and somewhat more particularly to quantitative analysis of elements in alloys, such as carbon in steel, using a direct reading spectrometer and a single calibration curve.
2. Prior Art
Direct reading vacuum emission spectrometers which employ at least one Internal Standard Line are known wherein the line is selected by the spectrometer manufacturers on the basis of particular design considerations and on the premise that the line is influenced in an identical manner to an element line being used for an analysis and that the background count for each analysis line is influenced by the matrix material. In other words, such spectrometers assume that those factors which influence the Internal Standard Line for a particular alloy equally influence all other elements of that alloy to the same extent and that while variations may be observed by changes in the integration time of the spectrometer, and although such times may vary, the analysis of the other elements remains unchanged.
However, the foregoing premise has been proven incorrect and the following demonstration will serve to illustrate the point.
Assuming that a spectrometer was set to integrate for 20 seconds on an original steel sample having a 98% iron matrix, then, for example, a 1% carbon content within this sample may result in a reading of 600 counts. Then, if 5% of the iron within the original steel sample were displaced by an equal amount of nickel so that an alloy or steel was obtained which was comprised of 93% iron, 5% nickel and 1% carbon and this modified alloy was analyzed with the above spectrometer, it would be noed that the count level determining capacitor takes longer to charge to the predetermined value since the Internal Standard Line (i.e., for iron) is of a lower intensity for the modified alloy. In other words, a longer integration time will result and the identical 1% carbon level will give rise to, for example, a reading of 700counts, which is incorrect.
Further, if the nickel within the above modified alloy were displaced by an equal amount of chromium and the further modified sample analyzed with the above spectrometer, prior art would assume that the identical (700) count would be obtained, since the iron content and thus the integration time remained unchanged. However, it has been ascertained that, despite the same integration time, the 1% carbon content in the above chromium-alloy will give rise to a lower reading, for example, 680 counts. This result is attained because the chromium content influences the carbon content readout and not the iron intensity.
In order to attain accurate measurements and in an attempt to overcome the lack of sensitivity in present spectrometers, as demonstrated by the foregoing, analysts have produced numerous calibration curves for various (standard) alloys, for example, a carbon calibration curve is available for 18/8 stainless steel, another curve is available for 24/20 stainless steel, etc. However, quite apart from the vast number of calibration curves required for standard alloys, off-specification alloys which are submitted for analysis cannot be accommodated.
In addition, instrument manufacturers have devised various ways, besides the Internal Standard method, for correcting the errors or inaccuracies resulting from changes in matrix content of alloys. However, these involve tedious mathematical calculations which are based on the matrix material content, for example, on the iron content in a steel, and are only suitable over limited ranges of matrix material content.